Quercus acorns as a component of human dietary patterns

1.1 Literature search strategy

A comprehensive search of the following academic databases was conducted: PubMed, Scopus, and Web of Science. The relevant keywords included mainly: “Quercus acorns,” “tannin removal,” “bioactive compounds,” “food scarcity,” and “gluten-free.” The studies that did not directly relate to the utilization of Quercus acorns were excluded. The quality and reliability of selected studies were based on the following factors: study design, sample size, methodology, statistical analysis, and ethical standards. The studies that included a peer-reviewed publication process and clear documentation of methods and results were prioritized.

1.2 Historical use of acorns

Acorns obtained from oak trees have been used for thousands of years in areas where oak trees naturally occur and bear fruit. Oaks can be found in Europe, Asia, North Africa, the Middle East, and North America, they belong to the Fagaceae family and the Quercus L. genus includes about 500 species worldwide [1,17]. The utilization of oak can be overseen also through the fact that oak timber has long been used for producing furniture, it is also used as biomass for energy production, cork from cork oak is used as insulation, and acorns used to be fed to domestic animals, especially pigs [18].

For example, bread in Sardinia used to be made from acorn flour, which was a source of nutrients and was more expensive than wheat bread due to higher laboriousness [19]. Kipping reports that acorns formed the basis of almost every dish of the Bay Area Indians, where California has about 20 species of oak trees with edible acorns, and a mature oak tree can produce 250–500 kg of acorns per season [20]. In the Mediterranean region, roasted acorns were also used as a substitute for coffee [18]. In Don Quoiote de la Mancha, acorn consumption is frequently mentioned and acorns are praised, suggesting that this food was popular in sixteenth-century Spain [18].

The main reason for limiting their use in current nutrition is their high tannin content; however, this can vary depending on the geographical origin, e.g., acorns from the south-east of Poland had significantly higher tannin content than those from the west [21]. The use of acorns in Europe declined after World War II. According to Pignone and Laghetti, society during that period was stratified by wealth, with food choices reflecting individuals’ economic status. Those who fed on acorns belonged to the poorest category, the richest category consisted of people who could afford classic white bread [18].

Currently, acorns are not considered a food in many countries, except for Native American and Korean cultures, for example, acorn jelly or acorn noodles are popular in Korea [18]. A feasibility study for acorn flour suggests that this flour could cost EUR 8–39/kg. However, the annual variability of the acorn harvest must also be considered, so risks based on this variable should be taken into account [14].

1.3 Nutritional value of acorns

The composition of acorns varies greatly depending on the species of oak from which the acorn comes, and the water content also plays a role (Table 1). On average, acorns contain 8.7–44.6% water, 2.3–8.6% protein, 1.1–31.3% fat, and 32.7–89.7% carbohydrates, the main carbohydrate being starch, which constitutes up to 55% of the dry weight of acorns [16].

Acorn flour is a significant source of minerals, containing 164 mg/100 g Ca, 1.04 mg/100 g Cu, 18.6 mg/100 g Fe, and 3.7 mg/100 g Mn, so this flour can be a good additive to enrich other flours with iron. Acorn flour contains up to 179 g/kg of fiber on a dry weight basis, making it an excellent source of dietary fiber [8,26]. Additionally, it is naturally gluten-free [26]. Among the gluten-free flours analyzed, acorn flour exhibited the highest concentrations of calcium, copper, and iron [8]. Acorn is also rich in phenolic compounds including flavonoids, although its content varies depending on the species. The highest content of phenolic compounds is usually associated with flour obtained from Quercus coccifera sp. The phenolic compounds are responsible for the browning of baked food products with the addition of acorn flour, due to oxidation caused by the activity of polyphenol oxidase [25]. The majority of the lipids of acorn belong to unsaturated fatty acids, with 61–63% of monounsaturated fatty acids and 16–17% of polyunsaturated fatty acids, while only 17–19% of lipid fraction consists of saturated fatty acids. Oleic and linoleic acids are the most abundant unsaturated fatty acids, while palmitic acid is the most abundant saturated acid in acorns [26].

Acorn is composed mainly of cotyledons, which do not have a starchy texture despite having a lower starch content. Instead, the oil content present in the acorns provides moisture and contributes to their overall texture. Nuts contain a protective layer that adheres to the kernel. This layer can contain up to 25% of the tannins and phenolic substances, and its taste tends to be astringent [12].

1.4 Tannins in acorns

Tannins are phenolic substances that consist of 3–5 carbons that bind to 2 or more protein molecules to form a compound. The name comes from its original use, tannins were used to tan leather when the protein structure of the leather was changed. Tannins can most often be found in the nut covers and also in unripe fruit, where the content is explained by protection against early consumption, when the seeds would not be ripe and viable; thus, such fruit is not attractive to animals due to its astringent taste. Tannins also act against bacteria and fungi by interfering with their surface proteins [12], and antiviral effects are also confirmed [27]. Tannins are present in the bark and acorns of various oak species [13]. Tannins can be quantified in matrices using the Folin–Ciocalteu method, where after removal of tannins by adsorption onto a solid matrix (polyvinylpolypyrrolidone), the content of tannins is subsequently subtracted from the content of total polyphenols; tannins can also be detected spectrophotometrically using phosphotungstic acid at a wavelength of 715 nm [13,28]. Tannin content can also be determined non-destructively, using near-infrared spectroscopy with the help of a calibration model from the spectra of individual acorn species [29]. Tannins are soluble in water and polar organic solvents, have high molecular weights (>500), complex and diverse polyphenolic groups, and can precipitate proteins [30]. For example, acorn nut flour differs from maize and potato starch due to its distinct tannin concentration and about 5% coarse fiber, 4% crude protein, and 6% crude fat [31].

Jolcham oak (Quercus serrata) contains an average of 6.4% tannins in the dry basis [32], while Turkey oak (Quercus cerris) contains up to 11.69% tannins in the dry basis [13]. The content of tannins in different parts of the oak (leaves) showed seasonal variability in the content of hydrolyzable tannins when their concentration dropped by more than half from the end of May to September [33]. Sun et al. reported that there are significant differences in the content of polyphenols, flavonoids, and tannins across oak populations [17]. Furthermore, acorn size was found to increase with increasing latitude, while polyphenol content decreased with latitude [17]. The trend of geographic influence on tannin content is also indicated by Łuczaj et al. When simulating the effect of burying acorns by stockpiling animals, it was found that acorns had a higher tannin content than fresh acorns 1 month after being stored in the ground [21]. The analysis of tannins after 3 months in the soil showed that the type of oak from which the acorn comes probably also plays a role, as the tannin content decreased in one case and remained almost the same in the other [24]. The high tannin concentration of acorns affects the flavor of foods made from them. Tannins can also combine with sugars and proteins to form a compound that slows down the body’s ability to absorb and digest fat, cellulose, protein, and carbohydrates [34].

The study showed that around 7.17% of the tannin in acorn nut flour may effectively prevent the enzyme proteins needed for the typical amylase breakdown of starch into fermentable glucose. Acorn nut flour contains roughly 50% carbohydrate by weight and a sizeable amount of tannin, which has a bitter taste and a depleting impact on nutrition. As a result, acorns are widely regarded as worthless bioresources and are mainly squandered in vast quantities every year. Additionally, some studies suggested that phenolic substances, like tannin, may inhibit important enzymes like amylase and amyloglucosidase [35].

Condensed tannins in acorns play significant ecological and nutritional roles, influencing both plant defense and animal nutrition. These polyphenolic compounds are synthesized through the phenylpropanoid biosynthesis pathway and are vital in deterring herbivory due to their astringency and protein-binding properties [36]. Condensed tannins in acorns vary significantly among oak species and influence their nutritional and digestibility profiles, particularly in ruminants. A study analyzing acorns from Quercus suber, Q. brantii, Q. coccifera, Q. cerris, and Q. infectoria found that condensed tannins contents ranged from 7.2 to 26.7 g/kg dry matter.

Among the studied species (in the work of Kaya et al.), Q. coccifera had the highest condensed tannin content, while Q. brantii and Q. suber exhibited relatively lower values. Condensed tannins’ role as an anti-nutritive factor arises from their ability to form complexes with proteins, reducing their digestibility. However, when polyethylene glycol, a tannin-binding agent, was added during in vitro analysis, gas production, organic matter digestibility, and metabolizable energy significantly improved across all species. This demonstrates that tannins negatively impact digestibility by limiting nutrient availability, which can be partially mitigated with polyethylene glycol [37]. The tannin content in different acorn species is given in Table 2.

Storage is preceded by harvest which begins in early September and in some cases may last until December, sometimes acorns may remain on the trees even throughout the winter [39]. Acorn flour from dried acorns has a low (approx. 18%) moisture content and is therefore better preserved [14]. Fungi of Alternaria spp. and Penicillium spp. are important pests during storage [40].

1.5 Mycotoxins

Acorns can be attacked by fungal spores after leaching and storage in unsuitable conditions, which can cause mummification, dehydration, blackening, and loss of nutritional value [41]. A study dealing with the occurrence of mycotoxins in acorns researched the effect of their occurrence at a specific distance from corn fields. Acorns collected at a distance greater than 90 m from the field were found to contain only vomitoxin. In addition to vomitoxin, acorns from forests and within 90 m of a cornfield also contained diacetoxyscirpenol, a trichothecene mycotoxin. Acorns located closer to the corn field were attacked by vomitoxin to a greater extent (82%) than acorns further away (60–67%) [42]. Kosicki et al. further reported that the occurrence of mycophenolic acid in optically moldy acorns was much higher (14,580 µg/kg) than in acorns without visible mold (85.4 µg/kg) mycophenolic acid [41]. Moreover, moldy acorns contained patulin (50 µg/kg) and zearalenone (4.9 µg/kg). Mycofenic acid is widely used as a component of immunosuppressive preparations in organ transplantation, and at the same time has low toxicity for ruminants [41,43]. The most common fungi in acorns were Penicillium expansum, Penicillium paneum, Penicillium crustosum, Penicillium aurantiogriseum, Mucor hiemalis, and Mucor circinelloides, with the Penicillium genus clearly dominant (97%) [41]. Due to the need to store acorns over the winter, it is necessary to protect them from adverse conditions where the occurrence of mold can lead to chronic diseases in animals and humans. Vasinauskienė et al., however, mentioned that acorns can be treated with hot steam to prevent molding and subsequent mycotoxin formulation. Specifically, a 14-s treatment helped to inactivate the genera of Penicillium, Alternaria, and Mucor, but this treatment can reduce germination, seedling height, and root diameter [40].

1.6 Tannins removal

The basis for preparing acorns contains 4 main steps: drying, grinding, leaching, and cooking. Leaching is done to remove bitterness, which is caused by the tannin content that also has anti-nutritional effects, such as blocking protein assimilation [10]. It has been proven that heat treatment can reduce the tannin content, specifically dry roasting acorns at a temperature of 200°C for 15 min was able to reduce the tannin content from 11.69 to 8.55% without affecting the overall antioxidant activity of the roasted products [13]. Before such roasting, the fruit cover should be cut to eliminate pressure accumulation and the resulting exploding of the product during roasting [7].

Cooking and leaching can improve the sweetness of acorns due to. Removal of astringent substances and tannins. Leaching takes place for several days during which the water must be changed regularly [14]. García-Gómez et al. found that the skin of the acorns was more often removed only after cooking because then the sub-skin membrane was easier to peel off [7].

Caps of the Q. cerris acorns can be used for targeted extraction of tannins. This extraction can be carried out using methanol:water mixture, where the amount of tannin obtained increases along with increased methanol: water ratio leaching duration up to 2–6 h [44].

Traditionally, the people in the area would identify and pick the trees that produced the sweetest acorns, which would then be utilized directly for food, while the acorns that were too bitter would be treated with the right amount of heat and either leached or detoxified using clay [45].

It should be added that according to the previous literature data, the maximum tolerable level of tannic acid intake, without causing harmful effects, is 800 mg/kg/day for rats and 560 mg for humans [46].

Tanin reductions from acorns are shown in Tables 3 and 4.